Houses military installations, a permanent Marine detachment and a small civilian population

Rich fishing grounds in surrounding waters

Scene of inter-Korean naval clashes in 1999 and 2002

In the 2002 exchange of fire, 13 Northern sailors and five Southern sailors were killed

North Korea’s reclusive leader Kim Jong-il is thought to be ill and trying to ensure the succession of his youngest son.

On Saturday, it emerged that North Korea had also shown off what it claimed was a new uranium enrichment facility to an American scientist.

The move prompted the US to rule out the resumption of six-party talks on nuclear disarmament that Pyongyang abandoned two years ago.

White House press secretary Robert Gibbs called on North Korea to “halt its belligerent action”, adding that the US was “firmly committed” to South Korea’s defence.

An emergency session of the United Nations Security Council could be held on Tuesday or Wednesday, a French diplomatic source said.

‘Unpardonable’

A spokesman for South Korea’s Joint Chiefs of Staff said North Korean shells had started falling in the waters off the island of Yeonpyeong at 1434 local time (0534 GMT).

At least 50 landed directly on the island, most of them hitting a South Korean military base there. Sixteen South Korean marines and three civilians were injured.

Analysis

Jonathan Marcus BBC diplomatic correspondent

Nobody needed any reminder of the volatility of the relations between North and South Korea, nor of the sensitivity of their disputed maritime border. In March, a South Korean warship was sunk by an explosion and an investigation indicated strongly that the North was responsible.

The shelling of Yeonpyeong fits into the same pattern. From the North Korean viewpoint, this is about establishing deterrence over the South and defending its interests. But it is also a wider demonstration to the world of the North’s power and an indication of some kind of political transition.

A resident on the island told the AFP news agency that dozens of houses had been damaged by the barrage. Television pictures showed plumes of smoke rising above the island.

“Houses and mountains are on fire and people are evacuating,” a witness on the island told YTN television station. “People are frightened to death.”

Local government spokesman Yoon Kwan-seok said the shelling lasted for about an hour and then stopped abruptly.

“All of the island’s 1,600-odd residents were evacuated to shelters,” he said.

The South’s military fired back some 80 shells. Casualties on the northern side are unknown.

South Korean fighter jets were also deployed to Yeonpyeong, which lies about 3km (1.8 miles) south of the disputed inter-Korean maritime border.

President Lee Myung-bak ordered the military to retaliate against North Korean targets in case of “additional provocations”, his spokesman said.

South Korean military officials later said that it had been conducting regular military drills in the sea off Yeonpyeong before the incident, but that no fire was aimed towards North Korea.

There was more condemnation of North Korea from Russia, EU and the UK, although China – the North’s main ally – refused to apportion blame.

A spokesman for the Chinese foreign ministry said that both countries should “do more to contribute to peace”.

“What’s imperative now is to restart six-party talks as soon as possible,” Hong Lei told a news conference in Beijing.

Japan’s Prime Minister, Naoto Kan, said he had ordered ministers to prepare for any eventuality.

Many residents of Yeonpyeong fled the island by ferry after the bombardment

“I ordered them to make preparations so that we can react firmly, should any unexpected event occur,” he said after an emergency cabinet meeting in Tokyo.

Russia’s foreign minister warned of a “colossal danger”, and said those behind the attack carried a huge responsibility.

The BBC’s John Sudworth in Seoul says news of the incident has caused concern on international financial markets, with both the Korean won and Japanese yen falling, but there is no sign of panic in the South Korean capital.

This western maritime border, also known as the Northern Limit Line, has been the scene of numerous clashes in the past.

In March, a South Korean warship went down near the border with the loss of 46 lives. International investigators say a North Korean torpedo sank the ship, although Pyongyang has denied any role in the incident.